Two new, and two ongoing projects are described in this proposal. The first is a study of water and solute transport across a variety of epithelia, utilizing new advances in the autoradiography of diffusible solutes. A broad series of problems in epithelial transport will be studied, including the function of heterogenous cell populations in the renal collecting duct, the nature of sodium-coupled anion transport in fish gill and choroid plexus, and the role of the paracellular pathway in transport. The effect of vasopressin on urea transport will be studied as well. Autoradiography will be carried out by a thin frozen technique that is particularly suited to the study of diffusible solutes. In general, the solutes studied will be labeled with tritium and 125I. The problems and tissues selected for study are those that give the best prospect of resolution of the labeled solute within the cellular compartment under consideration. The second project addresses the problem of solute transport across the peritoneum, with emphasis on the role of the mesothelial cells in determining the passage of large and small solutes. Permeability studies will be carried out in the autoperfused mesentery. This preparation will also be used for autoradiographic studies of the passage of large and small molecules across the mesothelial layer. Of particular interest is the determination of which solutes follow a transcellular pathway, and which follow a paracellular one. Two ongoing projects will be continued. Synthesis of prostaglandin by the collecting duct will be studied, in the light of recent unexpected findings with respect to prostaglandin synthesis in toad bladder in the presence of vasopressin and DDAVP. Using a method for the selective activation of water transport across the toad bladder, we will continue our study of the structure of the water channel.